Okay, I know that it is very hard for beginning students to understand the concept of why we use A, An, and The. So, I made up a poem to help them remember it by association. I taught my students that if you KNOW that there is supposed to be an eraser at the board, and it isn't there, you want to know where THE eraser is. And, if you want to know if I have an eraser you can borrow (but you don't know if I do or not), then you use AN. The poem is as follows:

If you know the answer, you must use THE (pronounced thuh).But if you don't know,use An or A (Pronounced uh).